Loads are frequently switched using what are known as high-side switches which are connected between the load and a circuit node which is at operating voltage. The load is therefore connected between the high-side switch and a circuit node which is at reference potential (usually ground). High-side switches are often used in preference to low-side switches connected between reference potential and load because, when high-side switches are used, the operating voltage is not applied to the load when they are in the off state. Often, transistor half-bridges comprising a low-side switching transistor and a high-side switching transistor are also used for switching loads, such as in power converters.
Suitable switches are, in principle, all kinds of transistors and thyristors. In the field of the power electronics, power MOSFETs and IGBTs are used as switching transistors. In contrast to logic circuits, where transistor half-bridges are frequently manufactured in CMOS technologies, NMOS transistors are preferred for switching loads in power due to their lower turn-on resistance.
However, actuating the high-side transistor in pure NMOS (and also in NPN bipolar) half-bridges requires a voltage which is higher than the operating voltage of the half-bridge and of the load, respectively. For this reason, driver circuits require a dedicated power supply for actuating high-side semiconductor switches. This can be provided, by way of example, by a capacitor which is charged via a diode when the high-side switch is off. Such an arrangement comprising a capacitor and a diode for supplying power to the floating (free of ground) driver circuit is also called a bootstrap circuit.
However, one problem is that such circuit arrangements comprising a high-side switch, a driver circuit for actuating the high-side switch and a bootstrap circuit for providing supply voltage to the driver are not capable of keeping the high-side switch in an on-state for as long as desired, since the capacitor is discharged by a current consumption, albeit a low one, in the driver circuit. Particularly in very-large-scale-integrated circuits, it is a complex matter to manufacture capacitors of appropriate size, and therefore the possible turned-on time of a high-side switch is limited by the size of the capacitor and the current consumption of the driver circuit.
However, there are applications in which the limited turned-on time of high-side switches is troublesome, and there is a need for high-side switches which can be kept in an on-state for an arbitrarily long time. Circuits are known which ensure that the driver circuit of the high-side switch is supplied with power permanently, e.g., with the aid of DC/DC converters. Such circuit arrangements are usually very complex, however. There is therefore also a need for a simple and inexpensive circuit for permanently supplying power to driver circuits for high-side switches.